(a) Field of the Invention
The present invention generally relates to a hybrid vehicle. More particularly, the present invention relates to a control system and a method for a hybrid vehicle which may optimally control the operating point of a vehicle.
(b) Description of the Related Art
A hybrid vehicle uses an engine and a motor as power sources. By selectively using the engine and the motor as the power sources, hybrid vehicles can beneficially enhance energy efficiency and reduce exhaust gas.
Driving modes of the hybrid vehicle can be divided into an engine mode (driving by an engine), an EV mode (driving by a motor), and an HEV mode (driving by an engine and a motor simultaneously).
By appropriately harmonizing power sources of the engine and the motor leads, fuel efficiency can be enhanced.
In the HEY mode, the driver's requirements induce an operating point as an optimal efficiency of a system that is calculated by torques and speeds of the engine and motor. In particular, the driver's requirements may be calculated by the sum of optimal torque of a motor and optimal torque of an engine.
However, in abnormal conditions, such as in high SOC (State Of Charge) conditions (i.e., above a preset SOC), the operating point is compensated for discharging (discharging of the battery, and in low (i.e., below a preset value) SOC conditions, the operating point is compensated for charging (charging the battery). Thus, if a battery is in a low SOC and a vehicle is being driven in the HEV mode, engine power is delivered for charging the battery and for driving the vehicle simultaneously. For example, if the vehicle is driven in the HEV mode, in low speed on a hill in low SOC, the engine is operated in low RPM which results in bad vibration characteristics with high torque, and thus terrible vibration is generated.
In an attempt to solve this problem, a shift speed can be kept in a low shift speed and the engine RPM is increased so as to reduce the vibration characteristic. However, in these conditions, while the vehicle vibration may be reduced, the engine RPM may be higher than required thereby making the passengers feel uncomfortable.
Further, although the driver's requirements are calculated by the sum of optimal torque of a motor and optimal torque of an engine, the real output torque of the engine may be lower than the optimal output torque of the engine in low atmospheric pressure, for example in high altitude. Thus, the real output torque of the motor would need to be higher than the optimal output torque of the motor in order to satisfy the driver's requirements. As a result, the vehicle efficiency is deteriorated and the battery is excessively discharged.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.